Evaporation of solutions



March 26, 1935. E, MAN

l EvAPoRATIoN oF SOLUTIONS Filed Feb. 4, 1931 Patented Mar. 26, 1,935

UNITED STATES EVAPORATION OF SOLUTIONS Erik man, Stocksund, Sweden,assignor to Industrikemska Aktiebolaget,

Stockholm,

Sweden, a Swedish joint-stock company Application February 4,

1931, Serial No.v 513,296

In Sweden February 11, 1930 4 Claims.

In the manufacture of cellulose, that is to say of sulphite cellulose aswell as of sulphate cellulose, for instance, great quantities of heatescape at the drying of the cellulose. `This heat escapes from thedrying rolls in mixture with air, the temperature of this air-vapormixture being dependent on the quantities of air supplied to the dryingrolls. Conditions are much the same in paper mills. The quantities ofheat escaping from the drying rolls in the paper mills have been usedheretofore only to heat the air in so-called heat recovering apparatus.In the manufacture of sulphite as well as of sulphate cellulose there isa great need, however, for heat for the evaporation of the lyeswith thesulphate cellulose the so-called black lye, and with the sulphitecellulose the so-called sulphite waste lye.

The present invention substantially consists in the use of the said heatquantities for concentrating solutions by vaporization, such as the saidlyes. This may be eiected in the following manner, for example.

'Ihe air-vapor mixture escaping from the drying rolls is recovered, thesame being caused to deliver heat indirectly, through an intermediatepartition, to the lyes in question, in which process the air-vapormixture is cooled While the lye is being heated. The heat issubstantially obtained from the latent heat of the condensed vapor. Thelye thus heated is then cooled by evaporation of Water from the hot lyewith the aid of air, in a manner known per se.

Here, either the whole of the quantity of airvapor mixture rising fromthe drying rolls or, preferably, only a part or parts thereof may bemade use of, as will be described more fully hereinbelow.

The air-vapor mixture obtained in the first case has a comparatively loWtemperature of condensation, and for this reason it is not so suitablefor the purpose as it would be if 'the temperature of condensation weresomewhat higher.

The accompanying drawing illustrates an arrangement according to theinvention, by means of which an air-vapor mixture of higher temperatureis obtainable.

Here, 1 designates a number of drying cylinders having a board or' paperweb 2 running over the same.

The moisture of this web is lowered on the drying rolls successivelyfrom 5070% water to 8 or 10% water. It is evident that the airsurrounding the paper Web-in question absorbs Water therefrom, and thatsuch absorption may,'if the paper web holds a great percentage ofmoisture,

take place until the air has been saturated. On the other-hand, however,it is obvious that board or paper having a low percentage of moisture.such as l0-30% water, cannot give off water to the surrounding air sothat the latter Will become fully saturated. By reason of the fact thatthe fibres of the paper webare hygroscopic, the cellulose will retain acertain quantity of Water, even when it is surrounded by air which isnot saturated with steam.

In the drying of cellulose it is of importance, moreover, that thecellulose does not have a high temperature when the percentage of wateris low, that is to say at the dry end of the set of rolls, for in suchcase the quality of the cellulose will be impaired. On the other hand,no such disadvantages are involved in maintaining a high temperature inthe cellulose web; as long as the moisture of the latter is great, thatis to say at the wet end of the set of rolls.

'I'hese conditions are responsible for the impossibility to attain a4high temperature in the air-vapor mixture escaping from the dryingrolls,

if the air-vapor, the so-called Wet air, escaping from the various partsof the set of trolls, is mixed together. Only if the set of rolls bedivided, so that the Wet air from that part of the drying rolls wherethe moisture of the cellulose is still comparatively great,',is takenper se, will it be possible to attain a relatively high temperature ofthis wet air. This is illustrated diagrammatically in the drawing. Theairescaping from the wet portion of the set of rolls is removed througha screening housing 3, as at 4, without being mixed together with theair escaping from -the dry portion of the set of rolls at 5. If at thesame time a somewhat higher temperature is maintained in the celluloseweb, in the portion screened by the housing 3, a higher temperature willbe obtained in the air-vapor mixture escaping at 4. It is evident thatthis screening may be arranged in different ways, that is to say so thatit will surround a portion of the set of rolls of varying size, it beinglikewise possible to arrange a plurality of such screening means.

Obviously, the air escaping at 5 has absorbed a much smaller quantity ofWater than that absorbed by the air escaping at 4. For this reason it ispossible to use the air escaping at 5 as drying air in the portion ofthe set of rolls screened by the housing 3, in which case this air(which escapes at 5) is collected in a housing 30, and is blown, forinstance by means of a fan 31, through a conduit 32 and the air intakes,underneath the portion of the4 set of rolls screened by the housing 3,or is supplied to said portion in some other way. This involves theadvantage that small quantities of air only are required for the dryingprocess, and the further advantage that the heat contents of the wet airescaping from the mill can be better utilized for evaporating purposes.

Part of the air escaping at 5 may be employed for preheating of air,that is to say of the fresh air taken in through the openings 7. Thisfresh air may also be heated in a known manner by the use of ue gases orsteam, and/or by the airvapor mixture escaping from the lye evaporators,as later described.

The drawing shows diagrammatically how the air-vapor (the wet air)obtained in the drying process is used for evaporation. The air-vapormixture escaping at 4 is supposed, in accordance with the aboveconsiderations, to have a relatively high temperature, and is furtherassumed to be saturated. It will be understood that the high temperatureof the saturated mixture is due to the heat imparted thereto beforeand/or during the passage through the drier housing, as by the heatobtained from steam heated drying rolls, although the particular sourceof this heat is not important to the broader aspects of the presentinvention. With the aid of the fan 8, this gasvapor is conveyed throughlthe conduit 9 to the heat chamber 10 of the evaporating apparatus,Where it is cooled in a manner known per se, water being then condensedand drawn off at 22. The cooled air-vapor, the temperature of which isof course considerably lower than it was at the intake into the chamberl0, escapes from the heat chamber, as at 11. This air may be used anewin the set of rolls and may be introduced at '7 or 6, or at both ofthese points, on having been heated, if desired. The heat given off bythe air-vapor has been absorbed by the solution taken in at 12 andflowing over the heat surface 23, preferably in counter-current againstthe air-vapor in 10. Thus the temperature of this solution will, when itescapes at 13, be considerably higher than it was when entering at 12.The hot solution obtained at 13 is introduced into an evaporator 14,whence it escapes through the pipe 15 to a pump 16, from which it isreturned to the intake 12 through the pipe 17, and is again introducedinto the heater. Air or other gas is introduced into the evaporator 14,as at 18, by means of a fan 33, and this air or gas is brought intocontact with the solution, preferably in coun tercurrent, from whichsolution water (solvent) evaporates by reason of the contactestablished. The air and the vapors forming escape at 19. A certainquantity of thinly liquid solution is supplied to the system, as at 2U,and a certain'quantity of concentrated solution is drawn off, as at 21.The evaporator 14, which is here shown as provided with a number ofperforated partitions 24 with overflow pipes 25 arranged therebetween,may also be of other known construction. The air-vapor mixture escapingat 19 may be employed as a deliverer of heat in an evaporating unit,such as that above described (consisting of a heater and an evaporator),but operating at a lower temperature interval. In such case this secondevaporating unit will correspond to a second effect in the ordinarymultiple-effect evapo rating systems. This air-vapor mixture may also beused to preheat the air taken in at 7 or 6, or at both of these points.

In order that the air-vapor escaping at 19 shall permit of beingadvantageously used as a deliverer of heat, it is necessary, however,that the temperature of this air-vapor (at 19) should not be'too low.This can only be attained if the air-vapor coming to the heat chamber10, that is to say the wet air from the drying rolls, has acomparatively high temperature.

What I claim isz- 1. In a plant for making cellulose or paper, thecombination of a drier including a housing and means for conveying thecellulose or paper therethrough, means for supplying air to the interiorof said housing for drying said cellulose or paper by direct contacttherewith, means for heating the air thus supplied to the drier, meansfor segregating the hot moist air produced at the dry end of said drierfrom that produced at the wet end, means for supplying the hot moist airproduced at the dry end of said drier to the wet end thereof forassisting in drying said cellulose or paper by direct contact therewith,a cellulose solution evaporator including a heating chamber havingtherein a heat transmitting wall, means for delivering the hot moist airfrom the wet end of said drier to said -chamber and passing the sametherethrough in heat exchanging relationship with said wall, means forcirculating cellulose lye over the opposite side of said wall wherebysaid lye is heated by condensation of moisture from said hot moist air,and means for conducting a gas in direct contact with the heated lye toconcentrate the same by evaporation.

2. A plant according to claim 1 including means for preheating the airbefore it is supplied to said drier housing, and means for recirculatingthe air escaping from the heating chamber of said evaporator throughsaid preheating means and drier.

3. A plant according to claim 1 including means for preheating the airbefore it is supplied to said drier housing, and means for supplying thegasvapor mixture resulting from direct contact of the gas with the lyein said evaporator to said preheating means to serve as the heatingmedium therefor.

4. A plant according to claim 1 including means for preheating the airbefore it is supplied to said drier housing, and means for supplying atleast a portion of the hot moist air produced at the dry end of thedrier to said preheating means to serve as the heating medium therefor.

ERIK MAN.

